Long Term Evolution (LTE) is the most recent step forward in cellular 3G services. LTE is a 3GPP standard that provides for an uplink speed of up to 50 megabits per second (Mbps) and a downlink speed of up to 100 Mbps. The LTE physical layer is a highly efficient means of conveying both data and control information between an enhanced base station (eNodeB) and mobile user equipment (UE). The LTE PHY employs Orthogonal Frequency Division Multiplexing (OFDM) and Multiple Input Multiple Output (MIMO) data transmission. In particular, the LTE PHY uses two types of OFDM schemes: Orthogonal Frequency Division Multiple Access (OFDMA) on the downlink (DL) and Single Carrier—Frequency Division Multiple Access (SC-FDMA) on the uplink (UL). OFDMA allows data to be directed to or from multiple users on a subcarrier-by-subcarrier basis for a specified number of symbol periods. SC-FDMA is also referred to with the term Single Carrier—Orthogonal Frequency Division Multiple Access (SC-OFDM).
OFDM systems break the available bandwidth into many narrower sub-carriers and transmit the data in parallel streams. Each sub-carrier is modulated using varying levels of QAM modulation, e.g. QPSK, QAM, 64QAM or possibly higher orders depending on signal quality. Each OFDM symbol is therefore a linear combination of the instantaneous signals on each of the sub-carriers in the channel.
Each OFDM symbol is preceded by a cyclic prefix (CP), which is used to effectively eliminate ISI. Further, the sub-carriers are very tightly spaced to make efficient use of available bandwidth, with virtually no interference among adjacent sub-carriers (Inter Carrier Interference, or ICI). The OFDM symbol consists of two major components: the CP and an FFT period (TFFT). With a CP of sufficient duration, preceding symbols do not spill over into the FFT period; there is only interference caused by time-staggered “copies” of the current symbol. Once the channel impulse response is determined (using periodic transmission of known reference signals, referred to as pilot symbols), distortion can be corrected by applying an amplitude and phase shift on a subcarrier-by-subcarrier basis.
OFDMA is employed as the multiplexing scheme in the LTE downlink. In OFDMA, users are allocated a specific number of subcarriers for a predetermined amount of time. These are referred to as physical resource blocks (PRBs) in the LTE specifications. PRBs thus have both a time and frequency dimension. Allocation of PRBs is handled by a scheduling function at the 3GPP base station (eNodeB). LTE frames are 10 msec in duration. They are divided into 10 subframes, each subframe being 1.0 msec long. Each subframe is further divided into two slots, each of 0.5 msec duration. Slots consist of either 6 or 7 ODFM symbols, depending on whether a normal or an extended cyclic prefix is employed (also referred to as short and long CP respectively). The total number of available subcarriers depends on the overall transmission bandwidth of the system. The LTE specifications define parameters for system bandwidths from 1.25 MHz to 20 MHz in terms of Physical resource block (PRB) bandwidth and number of available PRBs. A PRB is defined as consisting of 12 consecutive subcarriers for one slot (0.5 msec) in duration. A PRB is the smallest element of resource allocation assigned by the base station scheduler. The transmitted downlink signal consists of NBW subcarriers for a duration of Nsymb OFDM symbols. It can be represented by a so-called resource grid. Each box within the grid represents a single subcarrier for one symbol period and is referred to as a resource element. Note that in MIMO applications, there is a resource grid for each transmitting antenna. Special reference signals are embedded in the PRBs. Reference signals are transmitted during the first and fifth OFDM symbols of each slot when the short CP is used and during the first and fourth OFDM symbols when the long CP is used. Note that reference symbols are transmitted every sixth subcarrier. Further, reference symbols are staggered in both time and frequency. The channel response on subcarriers bearing the reference symbols can be computed directly from the received reference symbols. Interpolation is used to estimate the channel response on the remaining subcarriers.
The LTE PHY can optionally exploit multiple transceivers at both the basestation and UE in order to enhance link robustness and increase data rates for the LTE downlink. In particular, maximal ratio combining (MRC) is used to enhance link reliability in challenging propagating conditions when signal strength is low and multipath conditions are challenging. MIMO is a related technique that is used to increase system data rates. In order to receive a MIMO transmission, the receiver determines the channel impulse response from each transmitting antenna. In LTE, channel impulse responses are determined by sequentially transmitting known reference signals from each transmitting antenna. For example, in a Lx=2 Transmitter×Lx=2 Receiver MIMO system, there are a total of four channel impulse responses (C1, C2, C3 and C4). Note that while one transmitter antenna is sending the reference signal, the other antenna is idle. Once the channel impulse responses are known, data can be transmitted from both antennas simultaneously. The linear combination of the two data streams at the two receiver antennas results in a set of two equations and two unknowns, which is resolvable into the two original data streams.
Single Carrier—Frequency Domain Multiple Access (SC-FDMA) is used for the LTE uplink as an alternative to OFDMA as used for the LTE downlink in view of a lower power consumption. The basic transmitter and receiver architecture is very similar (nearly identical) to OFDMA, and it offers substantially the same degree of multipath protection. In SC-FDMA, the underlying waveform may be considered essentially single-carrier.
FIG. 1 schematically shows a basic SC-FDMA transmitter/receiver arrangement. Note that many of the functional blocks are common to both SC-FDMA and OFDMA, thus there is a significant degree of functional commonality between the uplink and downlink signal chains. The functional blocks in the transmit chain of the uplink are:
1. Constellation mapper: Converts incoming bit stream to single carrier symbols (BPSK, QPSK, or 16QAM depending on channel conditions);
2. Serial/parallel converter: Formats time domain SC symbols into blocks for input to FFT engine;
3. M-point DFT (also referred to as FFT block): Converts time domain SC symbol block into M discrete tones;
4. Subcarrier mapping: Maps DFT output tones to specified subcarriers for transmission. The specified subcarriers are a subset of Nsc consecutive subcarriers within a frame, referred to as an allocation;
5. N-point IDFT: Converts mapped subcarriers back into time domain for transmission; and
6. Cyclic prefix and pulse shaping: Cyclic prefix is pre-pended to the composite SC-FDMA symbol to provide multipath immunity in the same manner as described for OFDM. As in the case of OFDM, pulse shaping is employed to prevent spectral regrowth;
7. RFE: Converts digital signal to analog and upconverts to RF for transmission.
The constellation mapper, serial/parallel convertor, M-point DFT and Subcarrier mapping may together be referred to as a modulator.
In a MIMO application, a constellation mapper, serial/parallel convertor and M-point DFT are provided for each incoming bit stream in parallel and the subcarrier mapping comprises mapping the DFT output tones of the different M-point DFTs over Lx transmitter antennas into so-called layers by a MIMO encoder block, whereby the DFT output tomes are mapped to the same specified subcarriers. In Multi-Users MIMO (MU-MIMO), the antennas are related to different UE terminals. In Single-User MIMO (SU-MIMO), the transmitted antennas are related to the same UE terminal.
In the receive side chain, the process is essentially reversed. As in the case of OFDM, SC-FDMA transmissions can be thought of as linear summations of discrete subcarriers. Multipath distortion is handled in substantially the same manner as in the downlink OFDMA system (removal of CP, conversion to the frequency domain, then apply the channel correction on a subcarrier-by-subcarrier basis). Unlike OFDMA, the underlying SC-FDMA signal represented by the discrete subcarriers is single carrier.
In the uplink, data is mapped onto a signal constellation that can be QPSK, 16QAM, or 64QAM depending on channel quality. FIG. 6 schematically shows constellation diagrams for QPSK (left), 16QAM (middle) and 64QAM (right). However, rather than using the QPSK/QAM symbols to directly modulate subcarriers (as is the case in OFDMA), uplink symbols are sequentially fed into a serial/parallel converter and then into an FFT block (for performing the M-point DFT) as shown in FIG. 1. The result at the output of the FFT block is a discrete frequency domain representation of the QPSK/QAM symbol sequence. The discrete Fourier terms at the output of the FFT block are then mapped to subcarriers before being converted back into the time domain (using an N-point IDFT, also referred to as IFFT). The final step prior to transmission is appending a CP.
FIG. 1 also schematically shows the receiver side REC. The functional blocks in the receive chain are:
1. RFD: Converts RF signal to digital signal.
2. Cyclic prefix removal: Cyclic prefix is removed to retain a received signal vector y associated with just the FFT period;
3. N-point DFT: Converts received signal vector y from the time domain used for transmission to the associated mapped subcarriers in the frequency domain;
4. Frequency-domain processor: to perform equalization in the frequency domain and to calculate statistics for use in the demodulator. In MIMO applications, demapping the multiple Rx receiver antenna streams is also performed by the frequency-domain processor. In MIMO applications, the demapping provides signal streams per layer;
5. M-point IDFT: Converts back to time domain symbol blocks;
6. Parallel/serial converter: Formats time domain SC symbols into blocks for input to FFT engine;
7. Decoder: decodes the SC symbols into a bit stream of information bits.
The M-Point IDFT, the parallel/serial converter and the Decoder may together be referred to as a demodulator. In MIMO-applications, an M-Point IDFT, a parallel/serial converter and a decoder is provided for each of the layers.
The Cycle Prefix removal and N-point DFT blocks performs cyclic prefix removal and Discrete Fourier transform (DFT) of antennas input samples for every OFDMA/SC-FDMA symbol. Each DFT output sample may be called sub-carrier. The DFT size N is determined according to the system bandwidth as defined in the standard.
FIG. 2 schematically shows further details of the frequency-domain processor FDPP of the uplink chain as shown in FIG. 1.
The frequency-domain processor FDPP at the receiver side is arranged to perform channel and noise estimation to obtain an estimated channel response and an estimated noise variance by correlating the DFT output with a known transmitted pilot sequence for each SC-FDMA symbol.
The estimated channel response is commonly indicated as a channel response matrix H and a noise variance matrix S. The channel response and the noise variance are estimated for each SC-FDMA symbol and each sub-carrier. The frequency-domain processor further comprises a frequency-domain equalizer arranged to equalize the DFT output in order to suppress the channel effect using the estimated channel response and the estimated noise variance. The equalizer may be arranged to equalize according to, for example, a Linear Minimum Mean Square Error criteria (LMMSE) or an equalization algorithm based on Interference Cancellation and Linear Minimum Mean Square Error criteria, such as according to Successive Interference Cancellation MMSE (SIC-MMSE) criteria or to Parallel Interference Cancellation MMSE (PIC-MMSE) criteria. These and other equalizer and criteria are known to the person skilled in the art and are thus not described in more detail here.
The shown prior art frequency-domain processor FDPP further comprise a constellation statistics calculator CSCP. The constellation statistics calculator CSCP may use the estimated channel response and the estimated noise variance to calculate the average power of the constellation as well as the average power of the interference as received after equalization and IDFT stage. The average power of the constellation may further be referred to as the constellation power or constellation energy K. The average power of the interference may further be referred to as the constellation noise variance or interference power U. The constellation power or constellation energy K and the constellation noise variance or interference power U may also be referred to as the constellation statistics.
The demodulator at the receiver side is arranged to use the constellation power K and the constellation noise variance U to decode the IDFT output to obtain information bits for each SC-FDMA symbol. The demodulator may hereto comprise a constellation demapper and a decoder. The constellation demapper and the decoder may form an integrated block, or may be provided as separate blocks. The constellation demapper may also be referred to with the term demapper. The demodulator may comprise a Turbo decoder. An example of a Turbo decoder is described for example in article Gilberto Berardinelli, Caries Navarro Manch'on, Luc Deneire, Troels B. Sørensen, Preben Mogensen, Kari Pajukoski, “Turbo Receivers for Single User MIMO LTE-A Uplink”, IEEE 69th Vehicular Technology Conf., April 2009, pp. 1-5 (further referred to as Berardinelli). The demapper uses K and U in order to calculate the log-likelihood-ratio metrics (LLRs) needed for the decoding. The demodulator may alternatively comprise a Maximum likelihood Euclidian based decoder (which is for example used in LTE in a so-called Reed-Muller decoder). Such Maximum likelihood Euclidian based decoder uses K in order to scale the normalized constellation hypothesis prior to calculating the distance between the IDFT output signal and a hypothesis signal. Various Turbo decoders, Maximum likelihood Euclidian based decoders as well as other decoders are well known in the art.
Optimal or conventional as well as different receiver algorithms have been described in the literature. For example, article Narayan Prasad, Shuangquan Wang, and Xiaodong Wang, “Efficient Receiver Algorithms for DFT-Spread OFDM Systems”, IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS, VOL. 8, NO. 6, JUNE 2009, p. 3216-3225 compares an alternative algorithm to the conventional algorithm. Most proposed receiver algorithms require a large amount of resources to be implemented.